Gases exhausted from internal combustion engines such as spark-ignited internal combustion engines and diesel engines (especially from diesel engines) include several substances, hereinafter referred to as “emission(s)”, such as NOx and particle matters (hereinafter referred to as “PM”). It is desirable to decrease the amount of the emissions as much as possible. Examples of methods to decrease the amount of the emissions in the exhaust gases includes a method where an EGR gas amount, which is an amount of gas recirculated with an EGR device, is controlled depending on operating conditions of the internal combustion engines.
On the other hand, there is a trade-off relationship between the amount of NOx in the exhaust gas and the amount of PM in the exhaust gas, as is known in this technical field. That is, the amount of PM will increase when the internal combustion engine is controlled so as to decrease the amount of NOx, or the amount of NOx will increase when the internal combustion engine is controlled so as to decrease the amount of PM. Therefore, it is desirable to control the internal combustion engine in consideration of both of the amount of NOx and the amount of PM from the viewpoint of overall decrease of the amount of the emissions.
Specifically, it is desirable to control the internal combustion engine so that the amount of NOx is adjusted to coincide with a predetermined target amount according to operating conditions of the internal combustion engine in order to decrease the amount of NOx as much as possible while suppressing increase of the amount of PM. For example, one of conventional control devices for internal combustion engines (hereinafter referred to as “conventional device”) obtains operating parameters such as concentration of oxygen in gas introduced into cylinder, oxygen concentration of intake air; pressure of the gas in the cylinder, cylinder pressure; amount of fuel injected in the cylinder; and timing of the injection of the fuel in the cylinder. And then, the conventional device is configured to estimate the amount of NOx by applying values of the operating parameters to a predetermined model for estimating the amount of NOx. Furthermore, the conventional device is configured to control the amount of EGR gas, etc., so as to adjust the estimated amount of NOx to coincide with a predetermined target value, NOx target value (for example, see JP2002-371893A).